1. Field of the Invention
The present invention relates to ceramic articles, and more particularly to ceramic articles having properties suitable for use in exhaust after-treatment applications, particularly diesel exhaust filtration.
2. Technical Background
Recently, much interest has been directed towards the diesel engine due to its efficiency, durability and economical aspects. However, diesel emissions have been scrutinized both in the United States and Europe, for their harmful effects on the environment and on humans. As such, stricter environmental regulations will require diesel engines to be held to the same standards as gasoline engines. Therefore, diesel engine manufacturers and emission-control companies are working to achieve a diesel engine which is faster, cleaner and meets the most stringent of requirements under all operating conditions with minimal cost to the consumer.
One of the biggest challenges in lowering diesel emissions is controlling the levels of diesel particulate material present in the diesel exhaust stream. In 1998 diesel particulates were declared a toxic air contaminant by the California Air Resources Board. Legislation has been passed that regulates the concentration and particle size of diesel particulate pollution originating from both mobile and stationary sources.
Diesel particulate material consists mainly of carbon soot. One way of removing the carbon soot from the diesel exhaust is through diesel traps. The most widely used diesel trap is the diesel particulate filter which filters the diesel exhaust by capturing the soot on the porous walls of the filter body. The diesel particulate filter is designed to provide for nearly complete filtration of soot without significantly hindering the exhaust flow. However, as the layer of soot collects on the surfaces of the inlet channels of the diesel particulate filter, the lower permeability of the soot layer causes a gradual rise in the back pressure of the filter against the engine, causing the engine to work harder. Once the carbon in the filter has accumulated to some level, the filter must be regenerated by burning the soot, thereby restoring the back pressure to low levels. Normally, the regeneration is accomplished under controlled conditions of engine management whereby a slow burn is initiated and lasts a number of minutes, during which the temperature in the filter rises from about 400-600° C. to a maximum of about 800-1000° C.
Cordierite, being a low-cost material, in combination with offering a low coefficient of thermal expansion (CTE), has been the material of choice in diesel exhaust filtration. To that end, porous cordierite ceramic filters of the wall-flow type have been utilized for the removal of particles in the exhaust stream from some diesel engines since the early 1980s. A diesel particulate filter (DPF) ideally combines low CTE (for thermal shock resistance), low pressure drop (for engine efficiency), high filtration efficiency (for removal of most particles from the exhaust stream), high strength (to survive handling, canning, and vibration in use), and low cost. However, achieving the combination of high filtration efficiency, high strength, and very low pressure drop has proven elusive with cordierite DPFs.
Conventional DPF design has thus required the balancing of several factors, including porosity, pore size distribution, thermal expansion, strength, elastic modulus, pressure drop, and manufacturability. Further, several engineering tradeoffs have been required in order to fabricate a filter having an acceptable combination of physical properties and processability. For example, increased porosity is often attainable through the use of coarser raw materials, the use of pore formers, and or lower sintering temperatures. However, each of these approaches is known to result in an increase in thermal expansion which can compromise the survivability of the filter in the desired application. In addition, the heat capacity and thermal conductivity of the filter generally decreases with increasing porosity, thus leading to higher temperatures and more severe gradients during service.
In order to optimize a filter, the relatively fine porosity which reduces the heat capacity and thermal conductivity should be minimized because these pores can inhibit permeability. However, in order to maintain sufficient filtration efficiency and strength, relatively large pores should also be minimized. Larger pores often have a poor degree of connectivity to one another which has been found to result in clogging of the pores by soot during filtration, causing an undesirable rapid increase in pressure drop with soot loading and corresponding decrease in filtration efficiency.
It would be considered an advancement in the art to obtain an optimized ceramic article, suitable for use in filter applications requiring high thermal durability and high filtration efficiency coupled with low pressure drop along the length of the filter. In particular, there is a need in the art for a ceramic article which possesses a pore microstructure characterized by a relatively high level of porosity, a relatively narrow pore size distribution, and a relatively low coefficient of thermal expansion. To that end, as described below, the present invention provides such bodies and method of making them.